Return the response payload in creation helpers

Closes #72 (closed)

Some helpers were returning IDs only from the response payload:

• create_sub_element
• create_entity
• create_metadata

arkindex_worker/worker/classification.py

@@ -132,6 +132,8 @@ class ClassificationMixin(object):
 
         self.report.add_classification(element.id, ml_class)
 
+        return created
+
     def create_classifications(self, element, classifications):
         """
         Create multiple classifications at once on the given element through the API
@@ -199,3 +201,5 @@ class ClassificationMixin(object):
                 logger.warning(
                     f"Couldn't save created classifications in local cache: {e}"
                 )
+
+        return created_cls